In recent years, a cellular radio telephone system adopting a digital system has been proposed as one radio communications system. This type of system transmits, in digitized form, communication information, such as a speech message between a base station and a mobile unit in addition to a control signal. This system has several advantages in that it is possible to improve privacy, to secure data/system compatibility, and to effectively utilize the radio frequency.
The digital radio communications apparatus including this type of system uses, for example, a .pi./4 shifted DQPSK system and is so constructed as, for example, set out below. FIG. 1 is a block diagram showing an arrangement of the modulator.
A transmission stream SD is converted by a serial/parallel conversion circuit (S/P) 1 into two data streams X.sub.k and Y.sub.k. Here, the transmission data stream is comprised of an NRZ signal of, for example, 0 to 5 V.
These data streams X.sub.k and Y.sub.k are differentially coded by a differential coding circuit 2 into the following: ##EQU1## where I.sub.k-1, Q.sub.k-1 : the amplitude of the coded data at one previous pulse time; and
.DELTA..phi.: an amount of phase variation. PA1 .omega.c: the angular frequency of a carrier wave, PA1 .PHI..sub.n : the absolute value of the phase corresponding to an n-th symbol period. PA1 .PHI..sub.n obtained through the differential decoding is represented as follows: PA1 .PHI..sub.n =.PHI..sub.n-1 +.DELTA..PHI..sub.n
The amount of phase variation, .DELTA..phi., is determined by the amplitudes of the input data streams X.sub.k, Y.sub.k as shown in FIG. 3. As the amplitude value of the coded data I.sub.k, Q.sub.k, one is selected out of five values, that is, 0, .+-.1, .+-..sqroot.2.
The coded data I.sub.k, Q.sub.k delivered from the differential coding circuit 2 are input to a mapping circuit 3. At each pulse time of the coded data I.sub.k, Q.sub.k delivered from the differential coding circuit 2, the mapping circuit 3 determines, based on the phase mapping position of the coded data I.sub.k-1, Q.sub.k-1 obtained at one previous pulse time, the phase mapping position of the coded data I.sub.k, Q.sub.k at this current pulse time.
FIG. 4 is a phase space diagram representing the phase mapping positions of the coded data I.sub.k, Q.sub.k delivered from the mapping circuit 3. As evident from this diagram, the mapping position is so determined that, at each pulse time, any of those positions indicated by .quadrature. in FIG. 4 and any of those positions indicated by .smallcircle. in FIG. 4 are so determined as to be alternately selected.
The phase mapping positions of the coded data MI, MQ have eight combinations: (+I, +Q), (0, +Q), (-I, +Q), (-I, 0), (-I, -Q), (0, -Q), (+I, -Q), and (+I, 0). These combinations are represented by (+I, 0, -I) and (+Q, 0, -Q). Here, +I, +Q show the positive positions on the I axis, Q axis, respectively, and -I, -Q show the negative positions on the I axis, Q axis, respectively.
The coded data MI, MQ delivered from the mapping circuit 3 is input to a roll-off filter 4 where they are subjected to low-pass filtering processing. The roll-off filter 4 is used to reduce an influence resulting from the interference between those codes generated on a transmission path. The roll-off filter 4 is comprised of such a transversal type FIR filter as shown, for example, in FIG. 2. Given that, as here, one symbol (i.e., transmission unit: 2 bits in the .pi./4 shifted DQPSK modulation system) is represented by 256 samples, it is necessary that the signal entering the filter 4 corresponds to one obtained by sampling one symbol into 1/256 parts. If the impulse response of the filter is 10 symbols, filtering is carried out such that each sample of input data in (i.e., in 10.times.256 samples)=2,560 samples is multiplied by a corresponding factor.
The frequency response .vertline.H(f).vertline. of the filter is represented by, for example, the following: ##EQU2##
Here, T represents a symbol period and a roll-off factor .alpha. is for determining a transition band and, for example, 0.35.
The coded data (base band signals) MFI, MFQ delivered from the roll-off filter 4 are input to a quadrature modulator 5 where the coded data MFI, MFQ are quadrature modulated to a transmit intermediate signal corresponding to a radio channel frequency. The modulated transmit intermediate signal SIF is supplied to a transmitting circuit, not shown, for a radio transmission to be carried out.
Although not shown, the quadrature modulator comprises an oscillator, phase shifter for 90.degree. phase-shifting the output of the oscillator, first multiplier for multiplying the output of the oscillator and MFI, second multiplier for multiplying the output of the phase shifter and MFQ and an adder for adding together the outputs of the first and second multiplies. With the outputs of the oscillator given by A cos(.omega.ct), the transmit intermediate signal SIF is represented by ##EQU3## Here, g(t): the pulse shaping function,
Such .pi./4 shifted DQPSK system, being used, can suppress the broadening of a signal band.
However, the conventional .pi./4 shifted DQPSK modulator presents the following problem.
That is, given that one symbol of the coded data is represented with 256 samples, serial data corresponding to the 256 samples is delivered via respective I and Q channels from the mapping circuit 3 for each symbol and enters the roll-off filter 4. As evident from FIG. 2, therefore, the roll-off filter 4, being comprised of the transversal type filter, must include a shift register 41 having 2,560 steps corresponding to the 256 samples, 2,560 multipliers and 2,560 gates (not shown), for supplying factors to the corresponding multipliers 42. Further, since such transversal filter is required for each channel, the number of elements involved becomes doubled. Therefore, the circuit configuration of the modulator becomes very bulky and, in view of the many gates involved, it has not been possible to expect the realization of a compact circuit configuration.
Generally, with the mobile radio communications system, such as the portable telephone set or cordless telephone set, one of the important tasks is to make the system compact and light in weight. The problem thus posed provides a great bar to realizing a small and light-weight communication system.
It is accordingly the object of the present invention to provide a .pi./4 shifted DQPSK modulator which can reduce the number of gates of a filter and, by so doing, largely reduce the size of a resultant circuit configuration.